A typical technique of applying an adsorbent to an air-purifying filter includes the methods of producing an adsorbent-containing sheet and then placing the surface of the sheet perpendicular to the flow of the air to be purified. However, practically no conventional filter fully satisfies the important characteristics with respect to air-flow resistance, adsorption characteristics or dust-removing performance. Although there are a variety of conventional techniques lowering the air-flow resistance, no manufacturing method is practical, particularly because the conventional method tends to be complex.
Japanese Patent Application Laid-Open No.10-99421 discloses a method of producing an adsorption sheet using adsorbent particles and a powdered thermoplastic resin, which includes the steps of mixing the powdered thermoplastic resin of 100 to 1000 μm and granular activated carbon and then spreading the mixture on a base sheet and fixing it by pressing. This method uses a relatively large powdered thermoplastic resin and thus has the problem that a large amount of the powdered thermoplastic resin is needed for a sufficient adhesive effect so that the air-flow resistance can be high. The method also has the problem that the powdered thermoplastic resin can be fused to form a film in the vicinity of the boundary between the base sheet, the activated carbon and the thermoplastic resin in the mixture so that the air-flow resistance can be higher.
Japanese Patent Application National Publication (Laid-Open) No.07-509656 (with no description of the base sheet-combined type adsorption sheet) discloses a method for reducing the air-flow resistance of a molded activated carbon product, which includes the steps of: preheating adsorbent particles to a temperature higher than the melting range of a powdered thermoplastic resin; then mixing the heated adsorbent particles and the powdered thermoplastic resin to form fine agglomerates of at least 15 mesh; using a screen to select specific agglomerates; molding the agglomerates into the desired open-shaped flat structure; heating the flat structure to a temperature higher than the melting range of the powdered thermoplastic resin; and then cooling it. According to the disclosure, the two separate heating steps before and after the mixing step are needed for the production of the agglomerates. Such steps are required to consume considerable amounts of energy and time, and for example, each step takes as much as 40 minutes. It is also suggested that the melt of the powdered thermoplastic resin can be compressed by the influence of their own weight of the agglomerates so that the product can fail to have good permeability and that a well-permeable product cannot be obtained without an improved process including the step of turning the molded product upside down during the step of fusing the powdered thermoplastic resin.
JP-A No.08-290055 discloses a method for reducing the time of manufacturing a low air-flow resistance adsorption sheet, which includes the steps of adding 15 to 70% by weight of water to a mixture of activated carbon and a powdered thermoplastic resin so that the binding between the activated carbon particles is weakened by steam generated in a hot pressing process and performing a continuous process to form a product with a reduced air-flow resistance. In the end, the step of producing a final dry sheet requires considerable amounts of heat energy for evaporating the water, and such a method can be high in manufacturing cost and cannot be so practical.
In terms of deodorizing performance, odor gases and poisonous gases are broadly grouped under neutral, acidic and basic gases. A non-polar adsorbent such as activated carbon can have enough ability to adsorb the neutral gas by physical adsorption activity but cannot be so effective in adsorbing the acidic or basic gas as it is. Thus, the activated carbon should be treated with an agent for increasing the acidic or basic gas-adsorbing effect.
Japanese Utility Model Application Laid-Open No.04-41718 discloses a method including the step of attaching an organic acid and an amine compound to an activated carbon-bearing sheet substrate, wherein the attached organic acid can enhance the effect of removing ammonia and gaseous amines and the attached amine compound can enhance the effect of removing aldehydes, hydrogen sulfide, mercaptan, and the like. In this method, the two agents, the acid and the base, can be brought in contact and react with each other to lose their activity, so that the originally designed level of the deodorizing performance cannot fully be attained.
JP-A No.06-39238 discloses an improved method on the problems, which includes the step of using a water-soluble binder to increase the viscosity of the solution and to separate the acidic gas-adsorbent attached to an activated carbon sheet from the alkaline gas-adsorbent attached to an air-permeable sheet, wherein the activated carbon sheet and the air-permeable sheet are combined to form a single adsorption sheet. In this method, the attachment of the acidic gas-adsorbent to a more inner part of the activated carbon sheet can cause contact of the acidic gas-adsorbent with the air-permeable sheet to which the alkaline gas-adsorbent is attached, so that the adsorbents can lose their activity.
JP-A No.2000-84339 discloses a method including the steps of sandwiching a mixed powder of an activated carbon particle layer and a powdered thermoplastic resin between two air-permeable sheets having a deodorizing function and then hot-pressing them. In this method, the powdered thermoplastic resin has a large size of 50 to 2000 μm. Such large resin particles can increase the direct contact area between the activated carbon and the air-permeable deodorant sheet. In a case where a certain chemical is attached to the activated carbon, therefore, the chemical-bearing activated carbon and the air-permeable deodorant sheet can interfere with each other so that the deactivation rate can increase. In addition, the surface of the chemical-bearing activated carbon cannot uniformly hold the powdered thermoplastic resin. In a case where different chemicals are attached to the activated carbon, therefore, they can interfere with each other so that the deactivation rate can also increase.
JP-A No.11-57467 discloses an adsorption sheet with an improvement on the above problems, which comprises:
a plurality of laminated units each comprising deodorant powder; and
a web comprising a connecting portion made of a hot-melt resin and an agglomerate resin portion, wherein the deodorant powder of the one laminated unit is fixed on one surface of the web via the agglomerate resin portion;
the deodorant powder of the another laminated unit is fixed on the other surface of the web via the agglomerate resin portion;
the adsorption sheet is a laminate type deodorant filter having at least two types of the deodorant powders; each laminated unit has only one type of the deodorant powder; and the units are laminated and integrated into a single piece. In this technique, however, the different types of the deodorant powders must be used for the different laminated units, so that the sheet having at least two different types of the deodorant powders must be thick. According to this technique, a pleated or corrugated filter unit formed of such a sheet cannot have a large number of ridges and cannot exhibit satisfactory performance.
In order to solve the above problems, the inventors have made active investigations on the base sheet for use, the powdered thermoplastic resin for use and manufacturing conditions and consequently have found a method that can produce, at low cost, an adsorption sheet with very low air-flow resistance, good deodorizing performance and good dust-removing performance. Also provided is an adsorption sheet that can offer its performance more effectively without interference between two or more agents even in a case where the two or more types of the agents are attached to a powdered adsorbent or in a case where the agents are attached to a base sheet and an air-permeable sheet.